The present invention relates to a metallurgical induction heating apparatus. More particularly this invention concerns a crubible for an induction heating apparatus.
An induction heating apparatus has a crucible that is lined with refractory material and that is externally surrounded by a heavy-duty electric coil. Metal or metal alloy to be melted is loaded into the crucible and alternating current is passed through the coil so that a moving electromagnetic field is formed with the crucible. Thus eddy currents are generated in the metal to be melted so that the metal heats up and eventually melts.
A considerable problem in such installations is that it is necessary to heat the metal contained in the crucible without heating the crucible itself. Thus when a standard crucible constituted as a large metallic vessel lined with refractory material is used it becomes very difficult to limit the heating effect to the metallic structure of the crucible. By using a very low frequency, between 1 and 5 Herz, it is possible to limit the electromagnetic heating of the crucible, while augmenting the heating of the metal by means of another heating means such as a direct electric arc or the like. Thus in such a situation the low-frequency field serves mainly to mix and circulate the metal in the crucible.
It is, however, much more desirable to use standard-frequency electrical energy at 50 or even 60 Herz. The heating effect in the crucible can be greatly reduced by forming the crucible of a multiplicity of separate wall elements separated by mortar-like webs of insulating material. Such a system can be used without auxiliary heating and without excessive heating of the crucible. Such an arrangement does, however, have the disadvantage that the crucible is extremely expensive and leak-prone.
It is also known, as for example shown in French Pat. No. 1,509,043 to form the crucible, at least in the region of the coil, of a multiplicity of separate non-magnetic metallic plates. Once again refractory cement is injected between these plates to hold the assembly together. Similar arrangements can be found in the crucible described in the French Pat. No. 1,534,905. Nevertheless crucibles of such type, on account of their compact metallic structure, have a relative short service life because of excessive heating when used above frequencies of 60 Herz.
French Pat. No. 2,100,553 shows several induction heating arrangements. One of these shows a crucible comprising an upper collar and a lower base pan. The collar and base pan are provided with outwardly projecting mounting rings that are vertically interconnected by a plurality of vertically extending tierods. These tierods are electrically insulated from each other and from the mounting rings so that they form an open circuit. Such an arrangement appears to have a better mechanical behaviour than the afore-mentioned ones. However, its technology makes difficult to change eventually defective or worn parts. Moreover, the presence, in some cases, of continuous metallic elements surrounding the refractory line can cause heating by eddy currents when the used inductive coil has the same axis as the one of the crucible, as it is usually the case.